When placed in a magnetic field, the nuclei of certain atoms (e.g., 1H 19F, 13C, and 31P) that have a net nuclear magnetic moment exhibit a phenomenon known as nuclear magnetic resonance. After being placed in a magnetic field, the nuclear magnetic moments of such susceptible atoms take up certain discrete orientations, each orientation corresponding to a different energy state. The difference in energy between these levels is proportional to the strength of the magnetic field, the proportionality constant being known as the magnetogyric ratio or gyromagnetic ratio.
Transitions between these differing energy levels can be induced by applying an oscillating magnetic field, (the B1 field), created by a RF coil whose axis lies at right angles to the direction of the ambient magnetic field. The frequency at which energy is absorbed by the nuclei is termed the resonant frequency; and this frequency will vary with the type of nucleus, each of which has its unique magnetogyric ratio and is proportional to B0, the strength of the magnetic field or flux density. For example, the protons (nuclear species 1H) in water experience a resonance effect when placed in a magnetic field strength of 7 T and exposed to a RF frequency of 298 MHz (megahertz).
It is important to understand that the resonant frequency of a particular nucleus is proportional to the local magnetic field at the site of that nucleus; and that the local field may be different from the globally applied field. The applied field induces electronic currents in atoms and molecules surrounding the nucleus; and these, in turn, create small shielding or screening magnetic fields which are superimposed on the applied magnetic field. The net or effective magnetic field is therefore slightly different from the applied magnetic field.
The extent of the difference is a function of the nature and position of the atoms and molecules surrounding a particular nucleus. Thus, nuclei of the same species which exist in different atomic and molecular environments will experience different effective magnetic fields, and therefore will resonate at different frequencies. The separation of resonant frequencies from a chosen reference frequency is known as the chemical shift; and for 1H nuclei is in the range of about 1 ppm to 10 ppm (parts per million). This effect is exploited to provide valuable information on the structure of molecules.
The effects of magnetic resonance have been developed to provide a system for non-invasive evaluation of the human anatomy via magnetic resonance imaging (‘MRI’), which is commonly utilized as a powerful diagnostic tool and clinical aid. Thus, magnetic resonance spectroscopy (‘MRS’) and magnetic resonance spectroscopic imaging (‘MRSI’) are useful in obtaining chemical analyses localized in the anatomy of a patient.
By means of a RF pulse sequence, a broad range of resonant frequencies can be used to excite nuclei in different chemical environments simultaneously. Then, as the nuclei relax their return signal, a free induction decay (‘FID’) is generated in the pick-up coil. The FID comprises a summation of individual sinusoidal signals, wherein each sinusoidal signal is characterized by an amplitude (measured in volts) and a frequency (measured in Hz).
The amplitude of a signal is proportional to the concentration of those affected nuclei in the sample, whereas the frequency is characteristic of the unique chemical environment of those nuclei. Then, a Fourier transformation of the FID is used to generate a spectrum, which has multiple peaks (or resonances) and resolves the signals arising from nuclei in different chemical environments along a frequency axis. This spectrum of signals provides much information about the concentration and structure of the targeted subject matter. For this reason, both spectroscopic data and visual images are commonly utilized as alternative or simultaneous modes of evaluation.